Why are the rankings of universities not changing? Why is the demographic composition of top universities the same? In this review, these questions are addressed based on physics. Although size matters, higher ranks do not correlate with bigger sizes. The higher ranks belong to universities that have more authors who receive more citations. Citations are a record of how ideas spread from the source to the whole globe, in accordance with the physics of the logistics S-curve phenomenon. The spreading occurs in three periods – slow, fast, slow – and the population served by each idea during its lifetime depends on the size of the first big channel that carries the idea. An idea from a famous university has a larger spreading territory around it than an idea from a lesser-known university. Creativity is key: rankings come from visibility through citations, and, in turn, visibility for an author is aided by the higher visibility of the university. The demographic composition of the top universities is the same: for instance, the percentage of female authors and authors of East Asian origin among the 200 most cited authors does not vary significantly over the 20 highest ranked universities.

Why does it feel that the time passes faster as we get older? What is the physical basis for the impression that some days are slower than others? Why do we tend to focus on the unusual (the surprise), not on the ever present? This article unveils the physics basis for these common observations. The reason is that the measurable ‘clock time’ is not the same as the time perceived by the human mind. The ‘mind time’ is a sequence of images, i.e. reflections of nature that are fed by stimuli from sensory organs. The rate at which changes in mental images are perceived decreases with age, because of several physical features that change with age: saccades frequency, body size, pathways degradation, etc. The misalignment between mental-image time and clock time serves to unite the voluminous observations of this phenomenon in the literature with the constructal law of evolution of flow architecture, as physics.

Humans and technology are not in symbiosis. They are one species, not two. Humans, enveloped in artefacts of many kinds and ages (from writing, to airplanes), are evolving as one species, the ‘human & machine species’. This evolution is visible and recorded in our lifetime. Here, I illustrate the evolution of the human & machine species by focusing on commercial aircraft, the cooling of electronics, and modern athletics, which is a special laboratory for witnessing the evolution of animal locomotion. I show that these evolutionary forms of flow organization are in accord with, and can be predicted based on the law of physics that governs evolution in nature, bio and non-bio: the constructal law. Evolution, life and the human & machine species are physics.

People like to say that energy and water are two problems, two vital commodities in short supply. Here I draw attention to the emerging literature and physics principle (constructal law) that provide the scientific foundation for sustainability. I show that the sustainability need is about flow: the flow of energy and the flow of water through the inhabited space. All the flows needed for human life (transportation, heating, cooling, water) are driven by the purposeful consumption of fuels. This is why the wealth of a country (the GDP) is directly proportional to the annual consumption of fuel in that country. This hierarchical organization happens; it is natural and efficient. Sustainability is the one-word need that covers all the specific needs. Sustainability comes from greater freedom in changing the organization – the flow architecture – that sustains life. Greater freedom to change the design (from water and power to laws and government) leads to greater flow, wealth, life and staying power, i.e. sustainability.

The paper summarizes results from an experimental study of buoyancy-induced motion and heat transfer in a horizontal rectangular cavity with the two vertical ends at different temperatures and the long horizontal walls adiabatic. The cavity height/length ratio is A = 0·0625. The high-Rayleigh-number range reported on in this paper, 2 × 108 < Ra < 2 × 109, has not been studied before. It is shown that, contrary to lower-Rayleigh-number behaviour known previously, the core flow structure is non-parallel and is dominated by horizontal intrusions flowing along each of the two insulated horizontal walls of the enclosure. The fluid embraced by the two horizontal jets is practically stagnant and thermally stratified. Flow visualization experiments suggest that adjacent to the two horizontal jets two secondary flat cells are formed by the baroclinic pressure field in an analogous way to what is observed in intrusions in a stratified fluid. Nusselt-number-Rayleigh-number results for the overall end-to-end heat transfer in the horizontal direction are reported and compared with previous experimental and theoretical results available for lower Rayleigh numbers. It is shown also that the transition from a parallel core structure to one dominated by intrusion layers is governed by the parameter Ra½A, with Ra¼A < 1 as necessary condition for a parallel core flow.

The paper presents a theoretical and experimental study of natural convection in a horizontal cavity which communicates laterally with a large reservoir. The cavity walls and the reservoir are at different temperatures. It is shown theoretically that the flow consists of a horizontal counterflow which penetrates the cavity over a distinct length. The penetration length is shown to be proportional to the cavity height and to the square root of the Rayleigh number based on cavity height and cavity-reservoir temperature difference. The validity of the theory is demonstrated on the basis of a flow visualization experiment. It is shown also that the Nusselt number for cavity-reservoir heat exchange is proportional to the square root of the Rayleigh number, and is relatively insensitive to the Prandtl number in the Pr range 0·7 to ∞. The energy-engineering applications of the lateral penetration flow are discussed.

This paper reports a fundamental study of the fluid dynamics inside a triangular (attic-shaped) enclosure with cold upper wall and warm horizontal bottom wall. The study was undertaken in three distinct parts. In the first part, the flow and temperature fields in the cavity are determined theoretically on the basis of an asymptotic analysis valid for shallow spaces (H/L → 0, where H and L are the attic height and length). It is shown that in the H/L → 0 limit the circulation consists of a single elongated cell driven by the cold upper wall. The net heat transfer in this limit is dominated by pure conduction. In the second part of the study, the transient behaviour of the attic fluid is examined, based on a scaling analysis. The transient phenomenon begins with the sudden cooling of the upper sloped wall. It is shown that both walls develop thermal and viscous layers whose thicknesses increase towards steady-state values. The criterion for the existence of distinct thermal layers in the steady state is (H/L)½RaH¼ > 1, where RaH is the Rayleigh number based on attic height. The corresponding criterion for distinct viscous wall jets is (H/L)½RaH¼Pr−½ > 1, where Pr is the Prandtl number. The third phase of this study focused on a complete sequence of transient numerical simulations covering the ranges H/L = 0.2, 0.4, 1; RaH/Pr = 10, 103, 105; Pr = 0.72, 6. The numerical experiments verify the flow features described theoretically in the first two parts of the study. The effect of thermal convection on the net heat transfer between the bottom and top walls is illustrated numerically. Finally, the transient numerical experiments show that in the present parametric domain the single-cell circulation pattern is stable with respect to the Bénard instability expected in fluid layers heated from below.

This paper develops an alternative approach to evaluating the arbitrary constants found in Gill's solution for the boundary-layer free-convection regime in a vertical rectangular enclosure. The new method consists of calculating the net upward flow of energy through the enclosure and setting it equal to zero near the top and bottom boundaries of the cavity. The present method takes into account the impermeable and adiabatic properties of the horizontal end walls. The overall Nusselt number derived on this new basis is shown to agree well with available experimental and numerical heat-transfer correlations.

The phenomenon of buoyancy-induced convection in an infinite porous medium with a concentrated heat source is studied analytically. The transient and steady-state temperature distribution and flow pattern around the source are determined using a perturbation analysis in the Rayleigh number based on the heat generation rate at the source. The first-order transient solution derived in the paper is valid for Rayleigh numbers less than 10. The transient flow pattern consists of an expanding vortex ring situated in the horizontal plane containing the source. The steady-state solution, valid for Rayleigh numbers of the order of 20 or less, reveals an upward flow pattern which becomes very intense near the source. The upward flow extends throughout the medium. Both solutions show that as the Rayleigh number increases the region situated above the source is effectively heated by natural convection in addition to direct heat conduction from the source.